1. Technical Field of the Invention
The present invention relates to cellular telephone networks and, in particular, to inter-exchange handoff of mobile station call communications by taking into account the service capabilities of candidate cells.
2. Description of Related Art
Cellular telephone networks support a number of known service capabilities. Such differing service capabilities relate to: voice coder support (e.g., no voice coder, VSELP, or AFR); voice privacy support (e.g., no voice privacy, voice privacy algorithm A); data privacy support (e.g., no data privacy, data privacy algorithm A); hyperband support (e.g., 800 MHZ, 1900 MHZ, dual, and individual bands (A, B, and the like) therein); and voice/data communications support (e.g., analog, digital, facsimile, half/full rate, STU-III). It is not uncommon for the cells included within the network to support different ones of those service capabilities. Thus, for example, one cell may support voice privacy while another, perhaps adjacent, cell does not.
A cellular telephone call may implicate one or a number of these service capabilities. For example, an AFR voice coder only capable dual band (800/1900 MHZ) mobile station may establish a particular call in a cell using 1900 MHZ, AFR voice coder and voice privacy service capabilities. In order for this example call to be maintained as the mobile station moves and a hand-off of the call from cell to cell occurs, each target cell must be capable of supporting 800 MHZ or 1900 MHZ, AFR voice coder and voice privacy service capabilities (i.e., the same or acceptable alternate capabilities). A mobile switching center typically has knowledge of the service capabilities of each cell it serves, and thus making intra-exchange hand-off determinations can easily take into account difference in the service capabilities supported by each cell. That mobile switching center typically does not, however, have knowledge of the service capabilities of those cells it does not serve, and thus problems are encountered in making inter-exchange hand-off determinations to a target cell capable of supporting the same (or acceptable alternate) service capabilities as the currently handling cell. There is a need then for a technique for inter-exchange distribution of information concerning the service capabilities of individual cells. There is further a need for having this service capability information considered in the context of inter-exchange hand-off to ensure continued handling of cellular telephone call which require the use of certain ones of those capabilities.